Olefin polymerizations such as ethylene polymerization are frequently carried out using monomer, diluent and catalyst and optionally co-monomers and hydrogen in a reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. The product is discharged by means of settling legs, which operate on a batch principle to recover the product Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further discharged to a flash tank, through flash fines, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer is extruded and pelletized.
Multiple polyolefin reactors operating in series can be used for olefin polymerizations, as is known in the prior art. Certain polymerization processes comprise the use of two or several polymerization reactors, which are interconnected. A “bimodal olefin polymer” refers to an olefin polymer that is manufactured using two reactors, which are connected to each other in series. However, problems associated with known polymerization processes and apparatuses using a polymerization system having two or more serially disposed polymerization reactor vessels, include inaccurate inter-reactor transfer of polymer slurry between the serially disposed reactors, while maintaining each reactor at independently selected operating conditions. In certain cases, fewer fine particles (fines) are produced during transfer, which tend to hang-up or become trapped in transfer equipment and can even plug fines and valves. Frequent plugging causes system down time, lost final product and raw materials, and increased operating costs.
In the prior art systems, interconnected reactors have been described which are disposed in substantially vertical arrangements, i.e. reactors arranged in tandem vertical arrangement under an angle of inclination with respect to a horizontal axis extending from the exit of the first reactor which is more than 45°. Such arrangements require vertical product transfer lines or other vertical means for transferring polymer product from the polymerization zone of a first reactor to the polymerization zone of a second reactor. However, a problem associated with this type of configuration is that it requires the positioning of the reactors in a vertical arrangement, which is generally technically limited and results in increased fabrication costs. Also in such configurations the reactors are positioned dose to one another, which limits their accessibility.
U.S. Pat. No. 3,345,431 describes an apparatus for the co-polymerization of mono-1-olefins. The apparatus comprises two interconnected reactors. Transfer of polymer product from the first reactor to the second reactor is done by transfer of the polymer product into an auger conveyor which carries the polymer products from the first reactor to the wash column of the second reactor. An auger conveyor is a conveyor that has a trough or a tube in which a product moves under the action of an endless screw or flights. Thus, for transfer of the polymer product from one reactor to the other reactor a mechanical, motor-driven, device is used in the apparatus described in U.S. Pat. No. 3,345,431.
In view of the above, there remains a need in the art to provide a method and a polymerization reactor system in which operating problems experienced by prior art multi-reactor systems are reduced and in which the apparatus may be built and operated more economically than prior art systems.
It is therefore a general object of the present invention to provide multiple, interconnected reactors that are built and operated more economically than known prior art multiple reactors. Another object of the invention is to provide an improved method for production of polyolefins in general, and polyethylene in particular, in multiple interconnected reactors. A further object of the present invention is to provide an improved method utilizing multiple, interconnected reactors, which reduces construction and operating costs, and improves operating performance and operating versatility of the reactor system.